Preparation of propene and, if desired, 1-butene

ABSTRACT

Propene and, if desired, 1-butene are prepared by first reacting 1-butene and 2-butene to give propene and 2-pentene in the presence of a metathesis catalyst containing at least one compound of a metal of transition group VIb, VIIb or VIII of the Periodic Table of the Elements. The propene and 2-pentene formed are subsequently separated, and the 2-pentene is then reacted to give propene and 1-butene in the presence of a metathesis catalyst containing at least one compound of a metal of transition group VIb, VIIb or VIII. The propene and 1-butene formed are subsequently separated, and at least some of the 1-butene is discharged or at least partly isomerized to give 2-butene in the presence of an isomerization catalyst. Undischarged 1-butene and the 2-butene formed are subsequently returned to the initial reaction step, together with that part of the C 4  fraction which was not reacted in the initial reaction step.

CROSS-REFERENCES TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing propene and, ifdesired, 1-butene by metathesis of olefins.

BRIEF SUMMARY OF THE INVENTION

Olefin metathesis (disproportionation) in its simplest form describesthe reversible, metal-catalyzed rearrangement of olefins by cleavage andreformation of C═C double bonds. For example, olefins of the formulaeR¹—CH═CH—R² and R³—CH═CH—R⁴ are reversibly reacted to form olefins ofthe formulae R¹—CH═CH—R³ and R²—CH═CH—R⁴. In the metathesis of acyclicolefins, a distinction is made between self-metathesis in which anolefin is converted into a mixture of two olefins having different molarmasses and cross0- or co-metathesis in which two different olefinsreact. An example of self-metathesis is the reaction of two molecules ofpropene to give ethene and 2-butene, as is performed, for. example, bythe Phillips triolefin process, see Hydrocarbon Processing, Volume 46,November 1967, No. 11, p. 232. An example of cross-metathesis is thereaction of propene and 1-butene to give ethene and 2-pentene. If one ofthe reactants is ethene, the reaction is customarily referred to as anethenolysis.

The metathesis reactions are carried out in the presence of catalysts.Suitable catalysts for this purpose are, in principle, homogeneous andheterogeneous transition metal compounds, in particular those oftransition groups VI to VIII of the Periodic Table of the Elements, aswell as homogeneous and heterogeneous catalyst systems in which thesecompounds are present.

DE-A-19 40 433 discloses the metathesis of 1-butene with 2-butene toform propene and 2-pentene, with Re₂O₇/Al₂O₃ being used as catalyst. The2-pentene formed is reacted further with sodium hydride on potassiumcarbonate and ethene to give heptenes.

The metathesis of 1-butene and 2-butene to give propene and 2-pentene ismentioned in K. L. Anderson, T. D. Brown, Hydrocarbon Processing, Volume55, August 1978, No. 8, pp. 119-122 as a secondary reaction in thesynthesis of isoamylene.

EP-A-0 304 515 discloses a metathesis process for reacting 1-butene with2-butene to give propene and pentenes, which is carried out in areactive distillion apparatus using Re₂O₇/Al₂O₃ as catalyst.

U.S. Pat. No. 3,526,676 discloses the metathesis of 1-butene with2-butene to give propene and pentene. It is carried out over MoO₃ andCoO on Al₂O₃.

U.S. Pat. No. 3,785,957 discloses a process for the production of fuelhaving a high octane number. In this process, an olefinic fuel isdisproportionated together with ethylene, the product is fractionatedinto a propylene stream, a butene stream, a C₅- or C₅-C₆-olefin streamand a C₆₊ or C₇₊ fuel stream. The C₅- or C₅-C₆-olefin stream isdisproportionated with ethene over a WO₃/SiO₂ fixed-bed catalyst to givepropylene and butenes. The propylene obtained is disproportionated toform ethylene and butenes and the butenes are alkylated with isobutane.

U.S. Pat. No. 3,767,565 discloses a process for increasing the octanenumber of fuel in which a C₅ fraction of an olefinic fuel is reactedwith ethylene in the presence of a catalyst comprising WO₃/SiO₂ and MgOto form ethylene, propylene, n-butenes and isobutenes. The propyleneobtained is disproportionated and the resulting n-butenes are alkylatedwith isobutane.

EP-A-0 691 318 discloses an olefin metathesis process in whichC₅-olefins and ethylene are reacted in the presence of a catalyst togive mixed C₄-olefins and propene. Thus, 2-methyl-2-butene is reactedwith ethene to give isobutene and propene. A mixture of 2-pentenes and2-methyl-2-butene is reacted to give a mixture of 1-butene, isobuteneand propene.

2. Description of the Related Art

In the above processes, propene is prepared with addition of at leastequimolar amounts of ethene. To achieve high propene selectivities, alarge amount of ethene has to be circulated.

It is an object of the present invention to provide a process forpreparing propene and, if desired, 1-butene as coupled product invariable amounts using C₄-olefins such as raffinate II.

We have found that this object is achieved by a process for preparingpropene and, if desired, 1-butene by

a) reacting 1-butene and 2-butene to give propene and 2-pentene in thepresence of a metathesis catalyst comprising at least one compound of ametal of transition group VIb, VIIb or VIII of the Periodic Table of theElements,

b) subsequently separating the propene and 2-pentene formed,

c) subsequently reacting the 2-pentene with ethene to give propene and1-butene in the presence of a metathesis catalyst comprising at leastone compound of a metal of transition group VIb, VIIb or VIII of thePeriodic Table of the Elements,

d) subsequently separating the propene and 1-butene formed,

e) discharging at least some of the 1-butene formed and/or at leastpartly isomerizing the 1-butene formed to give 2-butene in the presenceof an isomerization catalyst and subsequently returning the undischarged1-butene and the 2-butene formed together with a part of the C₄ fractionnot reacted in step a) to step a).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 of the drawings schematically illustrates one embodiment of theprocess of the present invention by means of a flow diagram; and

FIG. 2 of the drawings schematically illustrates an additionalembodiment of the process of the present invention by means of a flowdiagram.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention comprises 2 metathesis steps. Inthe first step, 1-butene and 2-butene are reacted to give propene and2-pentene. In a second step, 2-pentene is then reacted with ethene to1-butene and propene. According to one embodiment of the invention, the1-butene is at least partially isomerized in the presence of anisomerization catalyst to give 2-butene and the resulting mixture of1-butene and 2-butene is returned to the first reaction step. A part ofthe 1-butene can be discharged before the isomerization or can beconveyed past the isomerization reactor and combined with the outputfrom the isomerization reactor. This, and also regulation of theconversion over the isomerization catalyst, can be used to set anoptimum ratio of 1-butene to 2-butene at the inlet of the firstmetathesis reactor in order to achieve a maximum yield of propene. Inthis way, the raw material requirements in terms of ethene andC₄-olefins can be reduced by from about 5 to 15% compared tosingle-stage ethenolysis processes as are described, for example, inU.S. Pat. No. 3,660,506 and EP-A-0 273 817.

According to one embodiment, the 1-butene formed in step c) is at leastpartially isomerized in the presence of an isomerization catalyst togive 2-butene and the resulting mixture of 1-butene and 2-butene isreturned to step a).

According to a further embodiment, at least some of the 1-butene formedin step c) is discharged and undischarged 1-butene is returned to stepa).

The net reaction is thus the reaction of 2-butene with ethene to form 2molecules of propene. According to one embodiment of the invention, thereaction of 2-pentene with ethene formally requires only equimolaramounts of starting materials in order to obtain the products in highyield. Thus, in contrast to the reverse triolefin process, the amount ofethene used can be kept small.

Both metathesis steps can be carried out as a reactive distillation, asis described below.

According to one embodiment of the invention, 1-butene and 2-butene canbe used in the reaction as pure substances. According to anotherembodiment of the invention, the butenes are used in the form of a C₄stream which originates, for example, from a cracker, in particular astream cracker, or a refining process. This C₄ stream can compriseC₄-alkanes in addition to the butenes. According to an embodiment of theinvention, use is made of a C₄ stream which consists of raffinate II.Raffinate II is here a fraction comprising 1-butene, cis/trans-2-butene,little or no isobutene and also n-butane and iso-butane. For example,raffinate II can comprise 80-85% by weight of olefins and 15-20% byweight of butanes, with, for example, 25-50% by weight of 1-butene,30-55% by weight of 2-butene and at most 1-2% by weight of isobutene.According to an embodiment of the invention, the C₄ stream used has abutene content of from 20 to 100% by weight, preferably from 50 to 90%by weight, in particular from 70 to 90% by weight. The ratio of 1-buteneto 2-butene is from 10:1 to 1:10, preferably from 3:1 to 1:3, inparticular 1:1. According to one embodiment of the invention, the C₄stream can contain small amounts of other hydrocarbons.

According to an embodiment of the invention, the starting material usedcan be any stream in which 1-butene and 2-butene are present. Accordingto one embodiment of the invention, the 1-butene can originate from thesynthesis of the present invention itself and be mixed with introduced2-butene.

The C₄ feedstream used is preferably pre-purified before use in theprocess of the present invention in order to remove any traces of water,oxygenates, sulfur-containing compounds or chlorides which may bepresent. The removal is preferably carried out by passing the C₄feedstream over absorber material such as aluminum oxide or molecularsieves, preferably NaX molecular sieve. The absorber materials arepreferably present as a guard bed.

Apart from the reaction of 1-butene and 2-butene to form propene and2-pentene, a small proportion of 3-hexene and ethene can be obtained asby-product. In addition, small amounts of higher-boiling compounds canalso be present.

The small amounts of by-products, which according to an embodiment ofthe invention make up from 1 to 30% by weight, preferably from 5 to 20%by weight, of the amount of 2-pentene formed, do not interfere in thesubsequent reaction so that, according to one embodiment of theinvention, no purification of the 2-pentene to remove these by-productsis necessary before the further reaction. According to one embodiment ofthe invention, the 2-pentene is used in pure form in the secondaryreaction.

Thus, the expression “2-pentene” also includes those mixtures comprisingnot only 2-pentene but also small amounts of hexenes, in particular3-hexene, and other higher-boiling compounds.

Correspondingly, the expression “butenes”, “1-butene” and “2-butene”also includes a mixture which comprises not only the butene or butenesbut also C₄-alkanes, in particular butanes.

A number of embodiments of the invention are illustrated below with theaid of the drawing, in which

FIG. 1 schematically shows an embodiment of the process of theinvention.

FIG. 2 schematically shows a further embodiment of the process of theinvention.

The abbreviations employed in the figures have the following meanings:

1-Bu: 1-butene 2-Bu: 2-butene Bu: butanes Et: ethene Pr: propene 2-Pe:2-pentene 3-He: 3-hexene H: high boilers II: raffinate II C4: C4 olefinsC5⁺: olefins having 5 or more carbon atoms R01: reactor (metathesis)R02: reactor (metathesis) R03: reactor (isomerization) K101:distillation column (preferably a dividing wall column, side column or2-column arrangement) K201: distillation column (preferably a dividingwall column, side column or 2-column arrangement) K301: distillationcolumn

Described below is a preferred embodiment of the process of theinvention, comprising

a) reaction of 1-butene and 2-butene to give propene and 2-pentene inthe presence of a metathesis catalyst comprising at least one compoundof a metal of transition group VIb, VIIb or VIII of the Periodic Tableof the Elements,

b) subsequent separation of the propene and 2-pentene formed,

c) subsequent reaction of the 2-pentene with ethene to give propene and1-butene in the presence of a metathesis catalyst comprising at leastone compound of a metal of transition group VIb, VIIb or VIII of thePeriodic Table of the Elements,

d) subsequent separation of the propene and 1-butene formed,

e) at least partial isomerization of the 1-butene formed to give2-butene in the presence of an isomerization catalyst and subsequentreturn of the 1-butene and the 2-butene formed together with a part ofthe C₄ fraction not reacted in step a) to step a).

This embodiment is shown in FIG. 1.

In a first reactor R01, 1-butene and 2-butene are reacted in thepresence of the metathesis catalyst of the present invention to givepropene and 2-pentene. For this purpose, a raffinate II stream is fed tothe reactor. The reactor is followed by a distillation column K101,configured as a dividing wall column, side column or 2-columnarrangement, at the top of which propene and ethene formed as by-productare removed. Unreacted raffinate II is taken off at the middle offtakeand some of it is returned to the feed stream of raffinate II. Some ofit may also be discharged. 2-Pentene and 3-hexene formed as by-productas well as high boilers are taken off at the bottom of K101. The bottomsare then fed together with added ethene to a reactor R02 which againcontains a metathesis catalyst of the present invention. In this reactorR02, the reaction of 2-pentene with ethene to give 1-butene and propenetakes place. The reaction product from reactor R02 is fed to adistillation column K201, configured as a dividing wall column, sidecolumn or 2-column arrangement, at the top of which propene andunreacted ethene are taken off. Some of the 1-butene formed may be takenoff at the middle offtake and at least some of it is preferably fed tothe isomerization reactor R03. Unreacted 2-pentene and also, asby-products, 3-hexene and high boilers are obtained at the bottom ofK201. These are discharged or preferably returned to R02. The mixturesof propene and by-product ethene taken off at the top of K101 and K201are fractionated in a further distillation column K301. Ethene isobtained at the top of K301 and this is preferably returned to thereactor R02. In the isomerization reactor R03, the 1-butene is at leastpartially isomerized to give 2-butene over an isomerization catalyst,and the isomerization mixture is returned to the reactor R01. The brokenline in R03 indicates the possible discharge of the 1-butene. Thepropene obtained at the bottom of K301 is, in addition to any 1-butenedischarged from K201, the desired reaction product of the process of thepresent invention. K101 and K201 are designed such that a low-boilingphase, in particular a C_(2/3) phase comprising ethene and propene, istaken off at the top of the column. C₄ streams, in particular butenesand butanes, are taken off as intermediate-boiling phase. As bottoms,C_(□5)-hydrocarbons are discharged.

Between the steps b) and c), the high-boiling product comprising2-pentene and 3-hexene which has been separated off can be subjected toa distillation to separate 2-pentene and 3-hexene. The distillation canbe carried out in any suitable apparatus. The 2-pentene-containingfraction is then fed to the reactor R02. The 3-hexene can be dischargedand, for example, fed to a dimerization to give a C₁₂-olefin mixture.

The reactors R01, R02 and R03 can be any suitable reactors. They canserve for continuous or batchwise operation. Thus, according to oneembodiment, they can be pressure vessels such as glass pressure vessels,while according to a further embodiment they can be tube reactors orreaction columns. Tube reactors are preferred.

According to an embodiment of the invention, the total conversion in R01is from 20 to 90%, preferably from 50 to 80%.

According to an embodiment of the invention, the total conversion in R02is from 20 to 100%, preferably from 60 to 90%.

The reaction in R01 preferably takes place in the liquid phase. Here,pressure and temperature are selected such that the reactants remain inthe liquid phase.

According to an embodiment of the invention, the temperature in R01 isfrom 0 to 150□° C., preferably from 20 to 80□° C. According to anembodiment of the invention, the pressure is from 1 to 200 bar,preferably from 5 to 30 bar. The reaction in R02 (ethenolysis) is,according to an embodiment of the invention, carried out at from 0 to150□° C., preferably from 20 to 80□° C., under an ethene pressure offrom 1 to 200 bar, preferably from 20 to 80 bar. Further ethene can beinjected continuously so that a constant pressure is maintained.

The reactions in R01 and R02 can be carried out for a time of from onesecond to ten hours, preferably from 1 to 60 minutes.

The distillation columns K101 and K201 are, according to an embodimentof the invention, columns which allow separation of a hydrocarbon streaminto C_(2/3) streams, C₄ streams and C₅₊streams. The columns can bedesigned as dividing wall columns, side columns or as 2-columnarrangements. According to an embodiment of the invention, K301 is acolumn which allows the separation of ethene and propene. According toone embodiment of the invention, the reactor R01 is combined with thedistillation column K101 to form a reactive distillation apparatus.Here, the reaction is carried out directly in the distillation column.The catalyst is present in the reaction column so that the distillationis carried out simultaneously with the reaction or immediatelythereafter. A corresponding process is known under the name “reactivedistillation”.

According to one embodiment, reactor R02 and distillation column K201are combined to form a reactive distillation apparatus in which thereaction and distillation are combined as in the above-describedreactive distillation.

According to one embodiment of the invention, both reactions take placein reactive distillation apparatuses. Both reactions are equilibriumreactions so that, according to one embodiment of the invention, theprocess products are removed as quickly as possible from the equilibriumto achieve as high as possible a conversion. This is possible, inparticular, when using reactive distillation apparatuses.

A further embodiment of the process of the present invention is shown inFIG. 2. FIG. 2 shows a process for preparing propene by

a) reacting 1-butene and 2-butene to give propene and 2-pentene in thepresence of a metathesis catalyst comprising at least one compound of ametal of transition group VIb, VIIb or VIII of the Periodic Table of theElements,

b) subsequently separating the propene and 2-pentene formed and theunreacted butenes,

c) subsequently reacting the 2-pentene with ethene to give propene and1-butene in the presence of a metathesis catalyst comprising at leastone compound of a metal of transition group VIb, VIIb or VIII of thePeriodic Table of the Elements,

d) subsequently transferring the unreacted mixture to step b) toseparate the propene and 1 -butene formed,

e) discharging at least some of the unreacted C₄ fraction separated offin step b) and/or at least partly isomerizing the 1-butene present inthis C₄ fraction to give 2-butene in the presence of an isomerizationcatalyst and subsequently returning the mixture obtained to step a).

As regards the reaction conditions, what has been said above in relationto the process as shown in FIG. 1 applies analogously. The mixtureobtained from the second metathesis reactor R02 is returned directly tothe distillation column K101. At least some of the intermediate-boilingproduct comprising C₄-olefins and butanes obtained from column K101 isdischarged and/or at least partly converted in the isomerization reactorR03 in which 1-butene is isomerized to 2-butene. The output from theisomerization reactor R03 is returned to step a), i.e. the metathesisreactor R01. In this process variant, the distillation column K201 canbe dispensed with.

Metathesis Catalyst

All suitable metathesis catalysts can be used in R01 and R02 in theprocess of the present invention.

According to an embodiment of the invention, the catalyst is aheterogeneous catalyst, in particular a supported catalyst. According toan embodiment of the invention, the catalyst comprises at least onecompound of a metal of transition group VIb, VIIb or VIII of thePeriodic Table of the Elements. The catalyst preferably comprises aruthenium compound and/or rhenium compound. Such catalysts aredescribed, for example, in K. J. Ivin, I. C. Mol, Olefin Metathesis andMetathesis Polymerization, 2nd Edition, Academic Press, New York, 1996;G. W. Parshall, S. D. Ittel, Homogeneous Catalysis, 2nd Edition, 1992,John Wiley & Sons, New York, Chichester, Brisbane, Toronto, Singapore,p. 217ff, R. H. Grubbs in Progr. Inorg. Chem., S. Lippard (ed.), JohnWiley & Sons, New York, 1978, Vol. 24, 1-50; R. H. Grubbs inComprehensive Organomet. Chem., G. Wilkinson (ed.), Pergamon Press Ltd.,New York, 1982, Vol. 8, 499-551; D. S. Breslow, Prog. Polym. Sci. 1993,Vol. 18, 1141-1195.

According to an embodiment of the invention, the metal compound is ametal oxide, partial oxide with additional organic radicals or acarbonyl compound.

According to one embodiment of the invention, a homogeneous catalyst isused. The catalyst is here at least one compound of a metal oftransition group VIb, VIIb or VIII of the Periodic Table of theElements. Preference is given to using rhenium or ruthenium in the metalcompounds.

The metal compound is preferably an oxide of rhenium, in particularRe₂O₇.

Support

According to an embodiment of the invention, the catalysts of thepresent invention comprise a support. Supports employed here are, inparticular, inorganic supports such as Al₂O₃, in particular □-Al₂O₃,SiO₂, Fe₂O₃, or mixtures thereof such as SiO₂/Al₂O₃, B₂O₃/SiO₂/Al₂O₃ orFe₂O₃/Al₂O₃.

The metal oxide content on the support is, according to one embodimentof the invention, from 1 to 20% by weight, preferably from 3 to 15% byweight, in particular from 8 to 12% by weight, based on the total weightof the supported catalyst.

The catalyst used is preferably Re₂O₇ on Al₂O₃, SiO₂/Al₂O₃,SiO₂/Al₂O₃/Fe₂O₃ or B₂O₃/Al₂O₃. The proportion of metal oxide here ispreferably from 1 to 20% by weight, particularly preferably from 3 to10% by weight. According to one embodiment of the invention, MeReO₃ isused in place of Re₂O₇ or in admixture therewith.

According to the present invention, particular preference is given tousing Re₂O₇ on Al₂O₃.

According to one embodiment of the invention, the catalysts are used infreshly calcined form and then require no further activation, forexample by means of alkylating agents. Deactivated catalysts can,according to the present invention, be regenerated by burning off carbonresidues, for example at 550□° C. in a stream of air and cooling underargon.

The reactions of the present invention can be carried out in thepresence of a solvent, for example a hydrocarbon solvent. According to apreferred embodiment of the invention, the reactions are carried outwithout further added solvent.

Isomerization Catalyst

As isomerization catalyst, it is possible to use all catalysts whichcatalyze the isomerization of 1-butene to 2-butene. Suitableisomerization catalysts are, in principle, any homogeneous orheterogeneous noble metal compounds, in the presence or absence ofhydrogen, as described, for example, in A. J. Hubert, H. Reimbinger,Synthesis 1970,1,405.

For example, the isomerization can be carried out as described in U.S.Pat. No. 3,592,868 over RuO₃ on an inorganic support such as SiO₂, Al₂O₃or TiO₂ or mixed supports. U.S. Pat. Nos. 4,684,760 and 4,889,840describe mixed catalysts comprising magnesium oxide, zirconium oxide andan alkali metal oxide on an inorganic support. EP-A-0 129 899 andDE-A-34 27 979 describe suitable phosphoric acids andphosphate-containing compounds and also zeolites of the pentasil type orzeolites doped with transition metals. The zeolites of the ZSM type,e.g. ZSM-22, ZSM-23 or ZSM-35, described in U.S. Pat. No. 5,177,281 areadvantageous in terms of operating life of the catalyst and reactionconditions. Particularly active palladium catalysts, for example onAl₂O₃ as support, are described in U.S. Pat. No. 3,531,545.

The isomerization catalyst is preferably a heterogeneous catalystcomprising one compound of a noble metal of the transition series of thePeriodic Table of the Elements which can be present in the form of themetal or an oxide or mixed oxide. Furthermore, compounds of a metal ofmain group I or II of the Periodic Table of the Elements are alsosuitable, and can be present as oxide or mixed oxide.

Preference is given to using a metal or metal oxide of transition groupVII or VIII of the Periodic Table of the Elements, which can be presenton a support, as isomerization catalyst in an inert gas atmosphere or inthe presence of hydrogen.

The alkali metal oxide and/or alkaline earth metal oxide catalysts whichare preferred according to the present invention are preferably preparedby impregnation of inorganic supports such as SiO₂, Al₂O₃, ZrO₂, TiO₂ ormixtures thereof with alkali metal and/or alkaline earth metalcompounds, subsequent drying and calcination to give the correspondingoxides. Deactivated catalyst can be regenerated in a simple way byburning-off coke residues at temperatures above 350° C. in a stream ofair and cooling in an inert gas atmosphere.

The isomerization catalyst which is particularly preferably used is PdOon an Al₂O₃ or SiO₂ support in the presence of hydrogen, where the Pdcontent is from 0.1 to 5% by weight, based on the total weight of thecatalyst.

When using the abovementioned catalysts, the C₄ intermediate-boilingfraction from the column K201 is converted by partial isomerization intoa mixture of 1-butene and 2-butenes and returned to the cross-metathesisin R01 to increase the propene yield. Alternatively, if the raffinate IIfeed stream has an appropriate purity, 1-butene can be obtained withoutfurther work-up. It can then be used, for example, for preparingpolymers such as LLDPE copolymers, HDPE copolymers, poly-1-butene or forpreparing butylene oxide.

In the isomerization, just as in the metathesis reactions in R01 andR02, the conditions are selected such that the reactants are present inthe liquid phase. The temperature is thus preferably from 0 to 200° C.,particularly preferably from 50 to 150° C. The pressure is preferablyfrom 2 to 200 bar. The isomerization is preferably complete after from 1second to 1 hour, preferably from 5 to 30 minutes. It can be carried outcontinuously or batchwise, and the reactors can, like the metathesisreactors, be glass pressure vessels, tube reactors or distillationcolumns. Here too, preference is given to using tube reactors.

The invention also provides an apparatus for carrying out the processdescribed. One apparatus for carrying out the process as shown in FIG. 1comprises a metathesis reactor (R01) for reacting 1-butene with 2-butenewhose outlet leads to a distillation column (K101), which can beconfigured as a dividing wall column, for separating C_(2/3)low-boiling, C₄ intermediate-boiling and C₅ ⁺ high-boiling phases, wherethe low boiler outlet leads to a column (K301) for separating ethene andpropene, the intermediate boiler outlet leads to the reactor (R01) or todischarge and the high boiler outlet leads to a reactor (R02) forreacting 2-pentene with ethene whose outlet leads to the column (K101)or to discharge, where the ethene outlet from the column (D3) and anethene feed line lead to the reactor (R2).

An apparatus for carrying out the process as shown in FIG. 1 comprises ametathesis reactor (R01) for reacting 1-butene with 2-butene whoseoutlet leads to a distillation column (K101), which can be configured asa dividing wall column, a side column or as a 2-column arrangement, forseparating C_(2/3) low-boiling, C₄ intermediate-boiling and C₅ ⁺high-boiling phases, where the low boiler outlet leads to a column(K301) for separating ethene and propene, the intermediate boiler outletleads to the reactor (R01) or to discharge and the high boiler outletleads to a reactor (R02) for reacting 2-pentene with ethene whose outletleads to a distillation column (K201), which can be configured as adividing wall column, a side column or as a 2-column arrangement, forseparating C_(2/3) low-boiling, C₄ intermediate-boiling and C₅ ⁺high-boiling phases, where the low boiler outlet leads to the column(K301), the intermediate boiler outlet leads to an isomerization reactor(R03) for the partial isomerization of 1-butene to 2-butene and, ifdesired, additionally to discharge, where the outlet of theisomerization reactor (R03) together with the intermediate boiler outletfrom (K101) leads to the reactor (R01) and the high boiler outlet leadsto discharge, where the ethene outlet from the column (K301) and anethene feed line lead to the reactor (R02).

EXAMPLES

Continuous Experiments on the Synthesis of Propene from Raffinate II

Example 1

Continuous Experiment on the Cross-metathesis of 1-butene with 2-butenein Raffinate II

Raffinate II (40% of 1-butene, 45% of cis/trans-2-butene) is passedcontinuously through a tube reactor R01 charged with Re₂O_(7/)Al₂O₃heterogeneous catalyst at 60° C., 25 bar and a space velocity over thecatalyst of 4500 kg/m² h. The reaction product is separated by means ofpressure distillation K101 (20 bar) into a C⅔ low-boiling phase, anintermediate-boiling fraction comprising C4-olefins and butanes and ahigh-boiling fraction comprising 2-pentene and 3-hexene. The percentagesreported are by mass:

C2/3 C4 C5/6 % m/m 21 56 23

Example 2

Continuous Experiments on the Ethenolysis of the C5/C6 High-boilingProduct

The high-boiling product comprising 2-pentene and 3-hexene from K101 ispassed continuously through a tube reactor R02 charged with Re₂O₇/Al₂O₃heterogeneous catalyst at 60° C. and 60 bar of ethene (starting materialratio C2:C5,C6=1.1:1) at a mean residence time of 5 minutes. Thereaction product is separated by means of pressure distillation K201 (20bar) into a C⅔ low-boiling phase, an intermediate-boiling fractioncomprising 1-butene and a high-boiling fraction comprising unreacted2-pentene and 3-hexene. The percentages reported are by mass:

C2/3 1-butene C5/6 % m/m 41 52 7

Example 3

Continuous Partial Isomerization of 1-butene

The intermediate-boiling product comprising 1-butene from column K201 ispassed continuously in the presence of 0.1 mol % of hydrogen through aflow tube charged with PdO/Al₂O₃ heterogeneous catalyst at 100° C. and15 bar at a mean residence time of 30 minutes. The reaction product isanalyzed by gas chromatography and consists of 60% of 1-butene and 40%of 2-butenes.

We claim:
 1. A process for preparing propene and optionally 1-butene bya) reacting a C₄ fraction of 1-butene and 2-butene to give propene and2-pentene in the presence of a metathesis catalyst comprising at leastone compound of a metal of transition group VIb, VIIb or VIII of thePeriodic Table of the Elements; b) subsequently separating the propeneand 2-pentene formed from the C₄ fraction in step a); c) subsequentlyreacting the 2-pentene from step b) with ethene to give propane and1-butene in the presence of a metathesis catalyst comprising at leastone compound of a metal of transition group VIb, VIIb or VIII of thePeriodic Table of the Elements; d) subsequently separating the propeneand 1-butene formed in step c); e) partially isomerizing at least someof the 1-butene from step d) in the presence of an isomerizationcatalyst to give 2-butene and returning the resulting mixture of1-butene and 2-butene to step a); and f) discharging the propane fromsteps b) and d).
 2. A process as claimed in claim 1, wherein at leastsome of the 1-butene formed in step c) is discharged and undischarged1-butene is returned to step a).
 3. A process as claimed in claim 1,wherein step b) is a distillation which is carried out in a dividingwall column, a side column, or a 2-column arrangement in which apropene-containing low-boiling phase is obtained, and/or wherein step d)is a distillation which is carried out in a dividing wall column inwhich a propene-containing low-boiling phase, a 1-butene-containingintermediate boiling phase and a 2-pentene-containing bottom phase areobtained.
 4. A process as claimed in claim 1, wherein the reaction insteps a) and/or c) is not carried to completion so that in step b)and/or d) a C_(2/3)-containing low-boiling phase, a C₄intermediate-boiling phase and a C+₅-containing bottom phase areobtained, wherein the low-boiling phases, which are combined, areseparated by distillation into C₂ and C₃ phases and the C₂ phase isreturned to step c), at least some of the intermediate-boiling phases,which are combined, are returned to step a) and at least some of thebottom phases, which are combined, are returned to step c).
 5. A processas claimed in claim 1, wherein 1-butene and 2-butene are used as amixture in a C₄ stream, from a cracker or a refinery, as raffinate II,and wherein the C₄ stream is passed over absorber materials before thereaction to purify it.
 6. A process as claimed in claim 5, wherein themolar ratio of 1-butene to 2-butene is from 10:1 to 1:10.
 7. A processas claimed in claim 1, wherein the metathesis catalyst is aheterogeneous metathesis catalyst comprising a rhenium or rutheniumcompound on an inorganic support.
 8. A process as claimed in claim 7,wherein the metathesis catalyst comprises Re₂O₇ on an Al₂O₃ support,with the rhenium oxide content being from 1 to 20% by weight, based onthe total weight of the catalyst.
 9. A process as claimed in claim 1,wherein the reaction of 1-butene and 2-butene and/or the reaction of2-pentene with ethene are carried out as a reactive distillation.
 10. Aprocess as claimed in claim 1, wherein the isomerization catalyst usedis a noble metal compound or its oxide, which is present on a support,in the presence of an inert gas or hydrogen atmosphere.
 11. A process asclaimed in claim 1, wherein, between steps b) and c), a high-boilingproduct comprising 2-pentene and 3-hexene, which has been separated off,is subjected to a distillation to separate the 2-pentene and 3-hexene.12. A process for preparing propene, which process comprises a) reacting1-butene and 2-butene to give propene and 2pentene in the presence of ametathesis catalyst comprising at least one compound of a metal oftransition group VIb, VIIb or VIII of the Periodic Table of theElements, b) subsequently separating the propene and 2-pentene formedand the unreacted butenes, c) subsequently reacting the 2-pentene withethene to form propene and 1-butene in the presence of a metathesiscatalyst comprising at least one compound of a metal of transition groupVIb, VIIb or VIII of the Periodic Table of the Elements, d) subsequentlytransferring unreacted mixture from step c) to step b) to separate thepropene and 1-butene formed, e) discharging at least some of theunreacted butenes separated off in step b) and/or at least partlyisomerizing the 1-butene present in this butene fraction to give2-butene in the presence of an isomerization catalyst and subsequentlyreturning the mixture obtained to step a), and f) discharging thepropene from steps b) and d).